The DS1685 /DS1687 are real-time clocks (RTCs) designed as successors to the industry standard DS1285, DS1385, DS1485 and DS1585 PC RTCs. These devices provide the industry-standard DS1285 clocking function with a +3.0V or +5.0V operating voltage. The DS1685 also integrates a number of enhancements including silicon serial number, power on/off control circuitry, 242 bytes of user NV SRAM and 32.768kHz output for sustaining power management activities. The DS1685/DS1687 power control circuit allows the system to be started by external stimuli (eg keypad) or time and date (wake-up) alarms. The PWR output pin can be triggered by one or either of these events and can be used to turn on external power. The PWR pin is under software control, so the system power can be turned off when the task is complete. DS1685 is a clock/calendar chip with the above functions. External crystals and batteries are the only components needed to maintain time and store state without power. DS1687 adopts DS1685 chip, 32.768kHz crystal oscillator and lithium battery, and adopts complete self-contained timing EDIP. The entire unit is fully tested at Dallas Semiconductor and guaranteed for a minimum of 10 years of timing and data retention without VCC.

figure 1

oscillator circuit

The DS1685 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or capacitors to operate. Table 1 lists several crystal parameters of the external crystal, and Figure 2 shows the functional schematic of a oscillator circuit. The oscillator is controlled by the enable bits in the control register. Oscillator start-up time is highly dependent on crystal characteristics, PC board leakage and layout. Excessive capacitive load with high ESR is the main reason for the long startup time. Suggested characteristics and proper layout for circuits using crystals usually start within a second.

Table 1

Power Saving/Power-Up Considerations

The RTC function continues to operate and all RAM, time, calendar and alarm memory locations remain non-volatile regardless of the VCC input level. . At least one standby supply must be kept within the minimum limit when VCC does not reach a valid level. When VCC is applied and exceeds VPF (power fail trip), the device is accessible after tREC if the oscillator is running and the oscillator is counting down the chain is not reset (register A). This time period allows the system to stabilize after power up. If the oscillator is not When enabled, the oscillator enable bit will be enabled on power-up and the device will become active immediately without any trouble. The DS1685/DS1687 are available in 3V or 5V devices. The 5V device is fully accessible and can only be written and written when VCC is greater than 4.5V Read data. When VCC is below VPF, reads and writes are disabled. However, the timing function continues to be unaffected by the lower input voltage. When VCC is below the greater of VBAT and VBAUX, the RAM and timer switch To a Lithium battery connected to the VBAT pin or the VBAUX pin. The 3V device is fully accessible and data can only be written or read when VCC is greater than 2.7V. When VCC is below VPF, reading and writing are disabled. If If VPF is less than VBAT and VBAUX, switch the power supply. When VCC drops below VPF, VCC is the backup power supply (the larger of VBAT and VBAUX). If VPF is greater than VBAT and VBAUX, when VCC drops to the larger value of VBAT and VB The power is switched from VCC to the backup supply VBAUX when VCC is below VPF. When VCC is below VPF, the device disables access by internally inhibiting the CS input. With the possible exception of the KS, PWR and SQW pins, all inputs are ignored and all outputs are in a high impedance state.

figure 2

main feature

Integrates the industry standard DS1287 PC clock and enhancements such as:

Y2K compliant

+3V or +5V operation

64-bit silicon serial number

Power control circuit supports system power on from date/time alarm or key off

32kHz output for power management

Crystal select bits allow the RTC to operate with a 6pF or 12.5pF crystal

SMI recovery stack

242 bytes of battery-backed NV RAM

Auxiliary battery input

RAM clear input

Century Registration

Date Alert Registration

Compatible with existing BIOS for original DS1287 functionality

Available with embedded battery and crystal chip (DS1685) or individually packaged DIP (EDIP) (DS1687)

Timing algorithm including leap year compensation valid until 2099